1. Field of the Invention
The present invention generally relates to washing apparatus, and more particularly, to a washing apparatus removing a contamination adhering onto a substrate. The present invention further relates to a washing method for removing a contamination adhering onto a substrate.
2. Description of the Background Art
When a film is formed on a wafer with a CVD method or a sputtering method, a particulate contamination adheres onto the surface of the wafer. Further, a resist residue sometimes adheres onto the surface of the wafer. As a method of removing these contaminations, a high pressure jet water washing method, a mega sonic running water washing method, an ice scrubber washing method and the like have been proposed.
FIG. 15 is a diagram showing a concept of an apparatus implementing a conventional washing method which is called high pressure jet water washing.
This apparatus includes a liquid pressurizer 3, a jet nozzle 4, and a stage 2 supporting and rotating a wafer 1. In this washing method, a liquid such as pure water compressed by liquid pressurizer 3 is continuously jetted out at a high speed toward the surface of wafer 1 by jet nozzle 4. The liquid jetted out at a high speed collides with the surface of wafer 1, whereby a contamination particle adhering onto the surface of wafer 1 is removed, and the surface is washed. Wafer 1 is rotated by rotating stage 2, and jet nozzle 4 is moved, so that the entire surface of wafer 1 is washed.
Referring to FIG. 16, the liquid jetted out at a high speed from jet nozzle 4 is formed into a liquid column 20 to collide with the surface of wafer 1 in this method.
This method has a problem as follows. Since a large amount of liquid such as pure water collides with the surface of wafer 1 at a high speed, static electricity is generated on the surface of wafer 1, which in turn damages devices formed on the surface of wafer 1. In order to reduce the damage, a method of mixing a CO.sub.2 gas or the like into the pure water to decrease the specific resistance of the pure water and to reduce static electricity generated on the surface of wafer 1 may be employed. However, this is not a perfect solution. Further, the method shown in FIG. 15 encounters another problem of not removing a small foreign matter (particle) of 1 .mu.m or less sufficiently.
FIG. 17 is a schematic diagram showing another conventional washing method which is called mega sonic running water washing. This washing apparatus includes stage 2 rotating wafer 1, and a nozzle 5 applying a high frequency of approximately 1.5 MHz to a liquid such as pure water and discharging the same. In this washing method, by vibrating the pure water at the high frequency in the vertical direction by nozzle 5 and jetting out the pure water toward wafer 1, the surface of wafer 1 is washed. This method encounters the following problems.
Similar to the case of the high pressure jet water washing method, a small foreign matter (particle) of 1 .mu.m or less cannot be removed sufficiently with this method. Further, although the washing effect is slightly increased in general by speeding up rotation of stage 2, the washing effect is large in the peripheral portion of wafer 1, while the washing effect is small in the center portion of wafer 1 in this case. Therefore, variation in washing occurs on the surface of wafer 1. Further, this method has a problem of destruction of miniaturized patterns of devices formed on the surface of wafer 1. It is recognized that there is a correlation between destruction of devices and the washing effect. In this washing method, the frequency and output of a high frequency to be applied, the number of rotation of stage 2, and the distance between nozzle 5 and wafer 1 are used as parameters controlling destruction of devices and the washing effect. However, it is difficult to control these parameters because of their small control ranges.
FIG. 18 is a schematic diagram of an apparatus implementing still another conventional washing method which is called ice scrubber washing. This washing apparatus includes an ice hopper 6 generating ice particles. Ice hopper 6 is provided with a supply spray 7 supplying pure water which is a liquid to be frozen to ice hopper 6. A jet nozzle 8 jetting out the ice particles toward wafer 1 is provided at the bottom of ice hopper 6.
Operation will now be described. A liquified gas such as liquid nitrogen is supplied to ice hopper 6. A liquid to be frozen such as pure water is sprayed into ice hopper 6 by supply spray 7. By jetting out the ice particles of several .mu.m to several tens .mu.m generated in ice hopper 6 toward wafer 1 from jet nozzle 8 of a gas ejector type, the surface of wafer 1 is washed. This washing method achieves a higher washing effect as compared with the above described high pressure jet water washing method and mega sonic running water washing method. However, the speed at which the ice particles are jetted out, which determines the washing force, cannot exceed the sonic velocity, because jet nozzle 8 of a gas ejector type is used. Therefore, there is a limitation in the washing force. Further, usage of a large amount of liquid nitrogen in order to form ice particles increases the initial cost of equipment for supplying liquid nitrogen, and the running cost is also high.
The above problem of destruction of devices can be suppressed by controlling the speed at which the ice particles are jetted out. However, in view of the constraints of assembling the apparatus, the speed at which the ice particles are jetted out can be currently controlled only within a range of 100 to 330 m/sec, and the control width is narrow. Therefore, destruction of devices cannot be suppressed completely.
Such problems in washing as described above also occur in the case where a contamination adhering not only onto the semiconductor wafer but also onto a liquid crystal substrate and a substrate of a photomask or the like is removed.